1. Field of the Invention
The invention relates to the production of ammonia. More particularly, it relates to an improved process and apparatus for forming ammonia synthesis gas.
2. Description of the Prior Art
The presently standard ammonia production technology is the process based on the steam reforming of natural gas or naphtha followed by a secondary reforming with air. Efforts have been made, however, to develop energy saving cycles, particularly in light of the drastic increase in energy costs that has occurred in recent years. Most of such developments do not employ pressure swing adsorption (PSA) technology for the purification of hydrogen to be employed in an ammonia synthesis gas stream.
The alternates to PSA-hydrogen purification include wash systems for carbon dioxide removal and methanation operations or a nitrogen wash for carbon monoxide removal. None of the alternate approaches provides for the complete removal of all contaminants and inerts from the hydrogen-containing gas to be purified in a single purification step. The potential for process simplification offered by the PSA-hydrogen purification process represents a desirable feature of this approach as compared with the alternates known in the art. This is of particular commercial significance since ammonia production constitutes the largest hydrogen consumer of all chemical processing industries. In addition to the growing market for ammonia, old ammonia plants are becoming obsolete, and a number of large new plants are being built to satisfy the demand for ammonia in more economical systems to offset the continually rising cost of energy.
The presently preferred feedstock for ammonia production is natural gas. Where natural gas is expensive or unavailable, naphtha is the next most preferred feedstock, but is found to be rapidly increasing in value, leading to a growing trend to base commercial plants on heavier petroleum fractions and coal. These feedstocks require oxygen for partial oxidation, generally with an oxygen supply from a captive air separation system. The nitrogen recovered from such a system can, of course, be employed in the overall ammonia synthesis operation.
It has heretofore been proposed to employ external source nitrogen as a purge gas in the purification of hydrogen used for ammonia production, as evidenced by Belgium patent No. 855, 126. As in conventional PSA processing, the external source nitrogen purge is utilized in such an approach at as low a pressure as possible, e.g., about 1.6 to 2 Bar absolute, so as to minimize the purge gas flow rate and compression power requirements. This has been consistent with the need to develop economical techniques for reducing the costs of producing ammonia synthesis gas.
It will be evident, however, that further improvements in the field of ammonia production are desirable in the art. In light of the high costs of energy, such improvements that may particularly enable energy costs to be reduced are especially desirable, and even necessary, if ammonia synthsis gas and ammonia are to be available at economical costs to satisfy growing industrial requirements.
It is an object of the invention, therefore, to provide an improved process and apparatus for the production of ammonia.
It is another object of the invention to provide a process and apparatus for the reduction of the energy costs associated with production of ammonia synthesis gas.
It is a further object of the invention to provide an improved PSA hydrogen purification system for use in the production of ammonia.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the approved claims.